say I'm going to cruise on the fwy at about 85~90

I can do this in 4th at about 4k or in 5th at about 3k. in 4th my turbos sleeping, but 5th wants about 5lbs. I've done a couple road trip experiments cruising in 4th or 5th and it seems like cruising in 4th gets better mpg. its close tho.. anybody know for sure?

you should be better mpg in 5th since it's @ a lower rpm...

thats what I thought, but then I again I think the boost is offsetting the rpm and actually drinking more gas

Originally posted by gordy
thats what I thought, but then I again I think the boost is offsetting the rpm and actually drinking more gas

Unless you are going uphill you should be running a vacuum while cruising at a constant speed, not positive boost pressure... Do you have a boost gauge?

I have a boost gauge. A constant 90 on the fwy in 5th gear tends to pull 2 to 5lbs.. it probably depends on how level the fwy is. I've never seen vacuum in 5th tho unless I'm actually slowing down. In 4th gear its vacuum for the most part unless I'm going noticeably uphill, but even then it definately pulls less boost than 5th

I guess the real question is whats more fuel efficient, rpms or boost?

I always knew it as 5th gear was the overdrive gear. Of course that's w/ NA engines, so I'm not sure w/ turbo'd engines. My guess is keep running your experiments for the good of all of us and let us know :).

Originally posted by wrex04
I always knew it as 5th gear was the overdrive gear. Of course that's w/ NA engines, so I'm not sure w/ turbo'd engines. My guess is keep running your experiments for the good of all of us and let us know :).

it's the same thing. 5th is overdrive (4th is slightly overdrive).. 5th lugs the engine a lot more than 4th which causes the turbo to work harder. my expirementing so far has been pretty sloppy so nothing conclusive.. but its close enough for me to wonder.

then again if 5th was that inefficient why would subaru gear it that way?

Originally posted by gordy
I have a boost gauge. A constant 90 on the fwy in 5th gear tends to pull 2 to 5lbs.. it probably depends on how level the fwy is. I've never seen vacuum in 5th tho unless I'm actually slowing down. In 4th gear its vacuum for the most part unless I'm going noticeably uphill, but even then it definately pulls less boost than 5th

I guess the real question is whats more fuel efficient, rpms or boost?

If I'm gentle on the throttle I'm pretty sure that I stay in vacuum in 5th, or at least very close to vacuum... I will check next time I'm on the fwy and report back...

interesting, find out and let us all know :D

man this is what I luv about the subie community. omfg i hate u all until i get my subie ;)

"say I'm going to cruise on the fwy at about 85~90

I can do this in 4th at about 4k or in 5th at about 3k. in 4th my turbos sleeping, but 5th wants about 5lbs. I've done a couple road trip experiments cruising in 4th or 5th and it seems like cruising in 4th gets better mpg. its close tho.. anybody know for sure?"



OK on my car 02 WRX crusing at 75MPH=3,000 RPM in 5th gear

You said 85~90??? Kph or MPH?

I asked subaru this question about a year ago and they told me that "the 2.0 WRX engine is most efficient running at 3,000RPM"
So 5th gear at 3000RPM, sure there is more wind drag while in 5th vs 4th. But when in 4th the trip takes longer and that uses more fuel than less time with slightly more wind drag...

How will a car have a higher coefficient of drag in one gear verses another? This makes no sense. Unless you are talking about "gear" drag.

hehehe I did this test when I first purchased my WRX Wagon and after the break in period. I am actually still doing the test but I have been testing for about a month now. This is how the test was done:

I drive the car on the beltway and the city but do not switch any lower than 4krpm. Second test I drive on the beltway and City but switch below 4krpm.

On the first test i got approx 18-19 mpg. On the Second test I got 20 mpg. I drive the car in the City at 4krpm but as soon as I hit the beltway and there are a normal amount of cars on the road I cruise in 5th gear. What makes this weird is that with my normal driving which includes beltway and city runs I get 19 mpg.

On the first test although I was in 4th gear at 70-80mph cruising the boost wast on until I tried to accelarate. The engine is cruising at approx 4500rpm but the response is better than 5th gear at 3krpm (obviously). The second test I was cruising at 70-80mph in 5th, approx. 3300rpm.

I also did a 3rd test which was drive the summama***** hard! in the City at 5krpm (i hardly used the brakes due to engine braking) and on the beltway at nothing lower than 4krpm. With that I averaged 18mpg at the most. I was expecting 16mpg but the lowest i got was 17mpg.

In conclusion: If you accelarate hard (using more boost) then you will lose alot of gas. If you accelarate moderately (not alot of boost) then hold in the high rpm (4k) you wont loss as much gas vs when on full boost. Boost is a MPG nightmare but a Performance fantacy... What a trade off.

Turbo cars are new to me and have a different characteristic vs N/A cars. In N/A cars the higher the RPM the more gas is used. In Turbo cars the case is the same but when in the boost is on the gas is sucked up alot regardless what the RPM.

Well thats my .02 and if you dont like it take this:monkey:

theworx...
Subaru told me 3000RPM gives the best efficancy. Thus 4th gear @ 3,000 yields LESS AIR DRAG than 5th gear @3,000 due to speed differances

I probally should have said that I get bewteen 27-29MPG mostly highway..

And also with the windows closed and the air conditioner on I get ONE MPG BETTER than with the air conditioner off and the windows open

Originally posted by gordy
say I'm going to cruise on the fwy at about 85~90

who cares about mpg, thats ticket territory, kind of outweighs the cost of gas

Originally posted by andyhidley
I probally should have said that I get bewteen 27-29MPG mostly highway..

And also with the windows closed and the air conditioner on I get ONE MPG BETTER than with the air conditioner off and the windows open

Just in case anyone is interested the speed at which you lose out on gas mileage from having your window open is around 40-45 mph. It depends on the frontal area of the car and how much drag will be created. Just incase anyone wanted some useless engineering facts. We did it in class a yr ago so just thought I would share some knowledge with the rest of you guys.

Originally posted by wrex04
Just in case anyone is interested the speed at which you lose out on gas mileage from having your window open is around 40-45 mph. It depends on the frontal area of the car and how much drag will be created. Just incase anyone wanted some useless engineering facts. We did it in class a yr ago so just thought I would share some knowledge with the rest of you guys.

how did you come to that conclusion? i am curious

andyhidley, I dont think anyone meant the same RMP in different gear...i think they were talking about the same speeds in different gear, which obviously would have the same drag

Originally posted by whitelegacy98
who cares about mpg, thats ticket territory, kind of outweighs the cost of gas

lol, yea.. I make the same trip about every other weekend from LA to sandiego and average about 85. 75 when there's cars, 85-90 when there's not. Between orange county and north county there's about a 30 mile stretch where around 2am I'm all alone. That's where I'll even top out around 135 for a bit :)

Well that's what I meant because the original question was what gives better fuel mileage, i.e. efficiency. Not constant speeds in differant gears. I think the bottom line is just use common sense.
Be easy on the throttle and try to run "under vacuume" not boost. Just because your running "under vacuume does not mean that the turbo isnt working. At 3000 RPMs you can cruise at 75 in 5th and go down a hill or on the level and have vacuume. But once you start going up a hill then you may move into the boost area.

I'm a graduating civil engineering major, however, in our fluids class we did lots of these calculations with cars. Since our prof was a guy who liked cars we spent a week on how to find real stats of these sorts of things. Like if a golf ball has dimples to improve the reynold's number and such would it benefit you to take a hammer to your car and make dimples? Anyhow, based on the drag equation it came out that on an average day that 40-45 mph is where having the windows down creates significant drag. These calculations were based on a volkswagon bug (the new one) b/c that's the shape and area he had. But i'm sure it's pretty comparable since they are both sub compacts.

Oh and w/ the dimples, your car is already designed to have a high reynold's number so no bashing up your car w/ some hail damage or ding's will not increase your speed. Only reason I say this is b/c a poll was taken in our class and half the class said yes it would help, but ppl don't do it b/c then our cars would look like crap. At least they were half right.

Originally posted by wrex04
I'm a graduating civil engineering major,

mechanical engineering is better, civil sucks


<--- has degree in mechanical engineering but works as a civil design engineer. go figure

i was originally wanting to know what equations and stuff you used to figure it out. but once you said your fluids class i figured out how you went about it.

Originally posted by andyhidley
theworx...
Subaru told me 3000RPM gives the best efficancy. Thus 4th gear @ 3,000 yields LESS AIR DRAG than 5th gear @3,000 due to speed differances

I see what you are saying now. I interpreted your meaning differently before.